With the advancement of electronic devices toward downsizing and low-profiling in recent years, there is a strong demand for reduction in size and thickness of push-on switches for use in the electronic devices while also providing superior electrical and mechanical characteristics. Description is provided of one such push-on switch of the prior art with reference to FIG. 4 and FIG. 5.
FIG. 4 is a cross sectional view of a conventional push-on switch, and FIG. 5 is a plan view of a switch base of the push-on switch shown in FIG. 4. Here, FIG. 4 illustrates a structure in which a complete assembly of the push-on switch is sectioned along the line 4—4 shown in FIG. 5.
In FIG. 4 and FIG. 5, switch base 301 made of an insulation resin is generally square in the exterior configuration as viewed from the above, and formed into a box-like shape with open-top recess 311 provided in the upper surface.
There are center stationary contact 302 and outer stationary contacts 303, all made of plated metals, and secured to the bottom of recess 311 by insertion molding.
Center stationary contact 302 has center contact point 323, which is exposed in the center of recess 311. Outer stationary contacts 303 have respective outer contact points 333 exposed in two positions along the outer brim of recess 311 with a predetermined distance of insulation spaces from center stationary contact 302.
Lead paths 321 and 331 branched out respectively from center contact point 323 and outer contact points 333 extend toward side edges of switch base 301 in an embedded form in the bottom of switch base 301, and the individual portions protruding outside from the side edges serve as terminals 322 and 332 for external connections.
Movable contact 304 is formed of a thin sheet metal into an upwardly convexed dome-like shape. This movable contact 304 is disposed in a manner that lower peripheral rim 341 thereof rests on outer contact points 333 of outer stationary contacts 303. Movable contact 304 is thus housed inside recess 311 in a manner that lower surface 342 at the dome-like top portion of movable contact 304 confronts the center contact point 323 of center stationary contact 302 with a predetermined space.
Recess 311 of switch base 301 is then covered with seal plate 305 made of a heat resistant plastic film such as polyimide, which is secured with adhesive to the upper surface of switch base 301.
Referring now to the plan view of the switch base in FIG. 5, description is provided in further detail of the arrangement of center stationary contact 302 and outer stationary contacts 303.
Recess 311 in switch base 301 is formed into generally a circular shape, as viewed from the above, in an area substantially the center of its square configuration. A single trace of lead path 321 is embedded and extends diagonally in one direction from center contact point 323 of center stationary contacts 302 set to be exposed at the center position on an inner bottom surface of recess 311. Lead path 321 protrudes outward from a side edge of switch base 301, and forms terminal 322.
Lead paths 331 extend individually from outer contact points 333 of the two outer stationary contacts 303 disposed along the other diagonal direction. The individual lead paths 331 extend and protrude outward from the side edges of switch base 301, and they individually form terminals 332.
The conventional push-on switch constructed as above operates in a manner which is described hereinafter. First of all, when a center portion of seal plate 305 is pushed from the upper side, a center portion of the dome-like movable contact 304 bends downward. If the pushing force on this dome-like center portion exceeds a predetermined amount, movable contact 304 is deformed into an inversed shape with a tactile response, and lower surface 342 of the top center portion comes in contact with center contact point 323 of center stationary contact 302. This completes a continuity between outer stationary contacts 303 and center stationary contact 302 through movable contact 304, thereby establishing an electrical turn-on mode.
When the pushing force impressed on the center portion of seal plate 305 is removed thereafter, movable contact 304 regains the original shape having the dome-like center portion in the upwardly convexed configuration with another tactile response by its resilient restoring force, thereby resuming the original state of electrical turn-off mode in which lower surface 342 of the dome-like top center portion comes apart from center stationary contact 302, as shown in FIG. 4.
The prior art documents known to be relevant to the present invention include Japanese Patent Unexamined Publication, No. 2004-119115, for example.
In the conventional push-on switch as described above, however, lead path 321 extends only to one side along the diagonal direction of switch base 301 from center contact point 323 of center stationary contact 302 disposed to the center of recess 311 in switch base 301, and lead paths 331 of two outer stationary contacts 303 are arranged at respective positions along the other diagonal direction, as shown in FIG. 5. For this reason, switch base 301 is subject to an adverse influence of heat when being connected with solder to a wiring board of an end use product. As a consequence, switch base 301 is likely to bear variations in expansion and shrinkage throughout areas where insert members such as lead paths 321 and 331 are embedded and corner areas where lead paths 321 and 331 are not present.